Process for continuously producing pure sorbic acid

ABSTRACT

Process for continuously producing pure sorbic acid from raw sorbic acid obtained by hydrolysis or thermal cracking of polyesters by crystallization from water, which comprises forming a saturated aqueous solution of raw sorbic acid at a temperature slightly below the boiling point of the water and with a short dwell time of the order of 2 to 5 minutes, separating the impurities from the dissolved sorbic acid by centrifuging at the same temperature, and crystallizing out the pure sorbic acid.

United States Patent [151 3,696,147 Kunstle et al. 1 Oct. 3, 1972 [54]PROCESS FOR CONTINUOUSLY [56] References Cited- PRODUCING PURE SORBICACID UNITED AT PATENTS [72] Invent: Gerhard Heinz -"1 3,510,514 5/1970Bordenca ..260/526 both of Burghausen upper Bavana, Ggrmany PrimaryExaminer-Vivian Garner [73] Assignee: Wacker-Chemie G.m.b.H., Munich,Atmmey ])ona1d Malcolm Bavaria, Germany [22] Filed: June 17, 1970 [57]ABSTRACT.

[21] Appl. No.: 46,878 Process for continuously producing pure sorbicacid from raw sorbic acid obtained by hydrolysis or ther- [30] FordApplication Priority Data mal cracking of polyesters by crystallizationfrom gn water, which comprises forming a saturated aqueous June 1969Germany 19 31 461-5 solution of raw sorbic acid at a temperatureslightly below the boiling point of the water and with a short [52] U.S.Cl. ..260/526 N dwell time of the order of 2 to 5 minutes, separating[51] Int. Cl ..C07c 51/42 the im tit-es from the dissolved sorbic acid bGem 581 Field of Search ..260/526 N P y trifuging at the sametemperature, and crystallizing out the pure sorbic acid.

1 Claim, 1 Drawing Figure PATENTEDflcrs 1912 INVENTOFES Gem-men KUNSTLE.

1 BY HE NZ. LIBERDH HTTORNEY PROCESS FOR CONTINUOUSLY PRODUCING PURESORBIC ACID The polyester obtained by reacting ketene and crotonaldehydein a known manner can be converted into sorbic acid in various ways,e.g. by hydrolysis or thermal cracking. In this manner, a polluted rawsorbic acid is always obtained whose by-products are formed during theconversion of ketene and during processing of the polyester.

It is also known that raw sorbic acid can be purified by known methods,for example by distilling, adsorption or crystallizing. v

A disadvantage of purification by distilling lies in the necessary useof carrier substances, for example steam (see-Japanese Pat. No. 478,361)or other reactioninert substances, e.g. hydrocarbons (see DAS Nos.1,059,899 and 1,252,664). Since the evaporation heat of the carriersubstance is necessarily lost, an additional consumption of energyarises in that case. Moreover, this method of purification requiresextensive apparatus, particularly when subatmospheric pressure is used.

The removal of impurities by adsorption, for instance on diatomaceousearth is suitable and economical only when the raw sorbic acid alreadyhas a high degree of purity. However, this is not the case with the rawsorbic acid made in accordance with known methods.

Even by crystallization from organic solvents, for example ketones oralcohols, or from a mixture of solvent and water, it is very difficultto remove the partly highmolecular and partly low-molecular by-productsfrom the raw sorbic acid. Moreover, the result is a sorbic acid which,with regard to color and stability, does not satisfy the highrequirements of purity. I

The disadvantages mentioned above do not occur during crystallizationfrom water. However, the separation of the by-products that are notsoluble in water causes difficulties, because the latter are either notretained by the filter or they quickly clog up the filter.

Attempts have already been made to remove the impurities by extractionwith an electrolyte-containing water and to isolate-the sorbic acid bycrystallization from a solution thus prepurified (see DAS No.1,181,203). The purification effect gets lower as the throughputincreases, so that only a low volume-time yield can be achieved. On theother hand, the necessary long period of contact between the impuritiesand the dissolved sorbic acid results in a dark coloring of the sorbicacid solution. Moreover, the periodic cleaning of the apparatusnecessary in this method represents an additional drawback.

According to DDR patent specification No. 48,119 raw sorbic acid ispurified by flotation. Because of the high volatility of the sorbicacid, losses of sorbic acid occur here as it is carried off. Additionalmeasures are required to prevent this. Also, the oily or tarry,specifically lighter layer (foam) obtained by flotation includes,besides water vapor, air or inert gases, -sorbic acid solution whoserecovery causes difficulties.

We have now discovered a process for the continuous production of puresorbic acid from raw sorbic acid obtained by hydrolysis or thermalcracking of polyesters by crystallization from water. The process ischaracterized by the fact that we make a saturated aqueous solution ofthe raw sorbic acid at a temperature slightly below the boiling point ofthe' water and with a short staying time, from this the impurities areseparated from the dissolved sorbic acid by centrifuging at the sametemperature, and the sorbic acid is crystallized out in the knownmanner. Contrary to the known purification methods, according to theprocess of the present invention all oily and tarry impurities of rawsorbic acid, no matter whether they are specifically lighter or heavierthan the sorbic acid solution, can be separated in a simple manner. Thisavoids a long period of contact between the sorbic acid solution and theimpurities which are primarily hard or impossible to dissolve in water,which surprisingly prevents almost all discoloration of the sorbic acidsolution and results in an improved quality of sorbic acid.

To avoid discoloration of the sorbic acid solution and secondaryreactions of the impurities with water with formation of water-solubleimpuritues it is useful to carry out the dissolving process atatemperature that is slightly below the boiling point of the water, e.g.to 99 C, keeping a short staying time, e.g. of the order of 2 to 5minutes. The staying time can be achieved by suitable measures, forinstance thorough mixing during the dissolving process. By this one canpractically eliminate the difficulties which arise due to the volatilityof water vapor of sorbic acid and its tendency toward decomposing.

Moreover, it has been found suitable to prepare a saturated sorbic acidsolution to increase the economy of the process by saving water. Herethe necessary quantity of water is referred to the pure sorbic acidcontained in the raw sorbic acid, whose content is determined, forinstance, by esterifying the raw sorbic acid with diazomethane anddetermining the sorbic acid methyl ester in the esterified mixture bygas-chromatographic analysis.

Suitable for separation of the impurities according to our invention areall processing machines which by their principle of effectiveness can beconsidered as centrifuges, and more exactly high-speed full jacketcentrifuges (see company article by Westfalia Separator AG,Oelde/West-phalia No. 3481/267, The

separation of liquid mixtures with the aid of separators, by H. Hemfort,Jr.). v

. The invention is described in connection with the accompanying drawingwhich is a diagrammatic illustration of an apparatus for carrying outthe process.

As illustrated in the drawing, into the mixing boiler 1, which isequipped with an efficient stirrer 2 and a heating jacket, raw sorbicacid is piped continuously through the pipeline 3. At the same timewater is piped into the boiler through pipeline 4 the water beingcirculated (mother liquor) and being heated to 98 C, in a quantity whichis sufficient to obtain a solution of sorbic acid which is saturated at98 C.

If necessary, concentrated caustic soda is piped in through line 5 toneutralize the hydrochloric acid contained in the raw sorbic acid in theform of diluted hydrochloric acid. The dwelling time of the componentsin the mixing boiler 1 is about 3 minutes at 98 C. During this periodthe raw sorbic acid is dissolved completely and one obtains an oily ortarry sorbic acid containing specifically lighter and specificallyheavier by-products, which is carried through line 6 into a separator 7for example, of the type Westfalia LG 205-3, which is heated to about 98C,. While the byproducts are continuously eliminated through line 8, thepurified, specifically lighter sorbic acid solution is withdrawn throughline 9 and it is carried to a crystallizing apparatus of any suitableknown design. The resulting sorbic acid crystals are separated in theknown manner, e.g. by a decanting centrifuge working continuously.

EXAMPLE Into the mixing boiler 1 we pipe in through line 3,200 parts byweight per hour of raw sorbic acid. Besides 0.25 percent hydrogenchloride in the form of a 2.68 percent hydrochloric acid, the liquidcontains 82.5 percent sorbic acid. At the same time we pipe in throughline 5 1.1 parts by weight per hour of concentrated caustic soda, andthrough line 4 8,250 volume parts per hour of water (98 C) whichcontains 2.0 g/l sorbic acid (circulation). Within about 2 minutes theraw sorbic acid is completely dissolved at 98 C under intensive mixing.The hot solution is continuously piped through line 6 into the separator7 which is maintained at about 98 C. While the oily or tarry impuritiesare carried off continuously through line 8, we obtain through line 9 aclear, almost colorless sorbic acid solution which is cooled to 30 C.Thereby sorbic acid is separated in the form of white crystals, whichare separated from the mother liquor and rinsed again with a little coldwater. After drying in a vacuum at 45 C one obtains 161.0 parts byweight per hour of sorbic acid which besides 0.3 percent water containspractically no other impurities.

The yield amounts to 97.3 percent, referred to the 100 percent sorbicacid used in the form of raw sorbic acid.

The mother liquor contains 2.0 g/l sorbic acid and can be used again fordissolving raw sorbic acid in circulation, or for processing.

The invention claimed is:

1. Process for continuously producing pure sorbic acid from raw sorbicacid obtained by hydrolysis or thermal cracking of polyesters obtainedby reacting ketene with crotonaldehyde by crystallization from water,which comprises, forming a saturated aqueous solution of raw sorbic acidby dissolving sorbic acid in water at a temperature ranging from to 99C/76O torr over a period of 2 to 5 minutes, separating the impuritiesfrom the dissolved sorbic acid by centrifuging at the same saidtemperature, and crystallizing the pure sorbic acid.

